The morphologies of human bones are measured to assay the growth and aging of bones, to diagnose and determine the degree of progress of bone diseases such as osteoporosis and osteomalacia, or to confirm a therapeutic effect.
Human bones are classified into cortical bones and cancellate bones. Cortical bones have dense bone structures having the shapes of pipes. Representative cortical bones are the shafts of the long tubular bones of the extremities.
Cancellate bones have mesh structures of osteocytes and are the epiphyses of long tubular bones, vertebrae, carpal bones, heel bones, anklebones, tarsi and such. Osteocytes of cancellate bones, as compared with those in cortical bones, have large areas in contact with soft tissues including blood vessels. Therefore, the progress of metabolism of cancellate bones is rapid and hence the progress of bone diseases in cancellate bones or changes in the state of cancellate bones caused by therapy are rapid.
MD, photon absorptiometry and radioscopy are used in generally known bone morphometric methods. MD measures density distribution in a roentgenogram of a sample bone, produced on an x-ray film by irradiating a sample bone with X-rays, by a microdensitometer (Kotsu Taisha, Vol. 13, 00. 187-195 (1980) and Kotsu Taisha, Vol. 14, pp. 91-104 (1981)), photon absorptiometry irradiates a sample bone with gamma rays and measures the quantity of transmitted gamma rays with a detector, and radioscopy irradiates a sample bone with X-rays and measures the quantity of transmitted X-rays with a detector.
MD is easily applicable to bone measurement and has progressively become prevalent because MD uses x-ray photographs that can be readily produced by the widespread x-ray photographic apparatuses which are widely used for diagnosing bone fractures.
These known bone morphometric methods, however, require drawing work for specifying a reference measuring line and a plurality of ROIs (regions of interest) near the reference measuring line on the x-ray photograph of a sample bone and it is difficult for even the same examiner to specify the same reference measuring line in examining the change in the sample bone and, consequently, accurate observation of the change in the sample bone cannot be achieved; that is, the ROI of the same sample bone cannot be accurately reproduced on each of a plurality of x-ray photographs of the same sample bone and, particularly with cancellate bones, the BMD (bone mineral density) varies widely with the variation of ROIs.
For example, in the SPA method (single photon absorptiometric method), in which a portion at a position along the bone equal to 1/6 of the length of the radius or 1/10 of the length of the ulna is measured, an operator measures the length of the radius or the ulna with a measure, puts a mark on the skin at a position corresponding to a measuring part, and then adjusts the forearm so that the forearm extends perpendicularly to a scanning direction in specifying a region of interest hereinafter referred to as an ROI ("Kotsu Mineraru Sokutei to Kotsu Soshyo Shyo", Medical Review K. K. (1989)). Therefore, the accurate reproduction of the ROI cannot be achieved due to errors in measuring the length of the radius or the ulna with a measure, and errors in determining the position to be marked, and this procedure needs a comparatively long time. Similarly, the drawing work for specifying an ROI on an x-ray film specifies a reference line by measuring the length of the radius or the ulna, and drawing a perpendicular on a center line connecting the head of the measured bone and the epiphysis of the same at a position at a distance equal to the measured length of the radius or the ulna, and uses the perpendicular as a reference line to specify an ROI. Errors in specifying the ROI entails an increase in the CV (Coefficient of Variance). The drawing work requiring a comparatively long time has been an obstacle to quick measurement.
In principle, because the patterns of the quantity of transmitted radiation obtained by the known bone morphometric methods include mixed information about the cortical and the cancellate bones, the known bone morphometric methods are unable to measure a region mostly including cortical bone and a region mostly including cancellate bone separately.
The inventors of the present invention have previously proposed, in Japanese Unexamined Patent Publication (Kokai) No. 4-84939, a bone morphometric method that reads an x-ray image of a sample bone formed on an x-ray film, comprising a process of obtaining a first smoothed pattern by obtaining density patterns of the sample bone along a plurality of substantially parallel measuring lines in a selected region of the input image and smoothing the plurality of density patterns at corresponding positions, and, if necessary, a process of obtaining a second smoothed pattern by smoothing the values of a plurality of nearby points along the measuring pattern in the first smoothed pattern.
However, when measuring the BMD of a cancellate bone by the conventional techniques, the measured BMD of the bone varies in a wide range due to the dislocation of image read lines for measurement for some kind of sample bones and for some measuring portions as shown in FIG. 1, which makes repeatable bone measurements difficult.
The conventional techniques are intended principally to measure the long tubular bones, such as the middle portion of the second metacarpus. The conventional technique determines the axis of a sample bone, i.e., the center axis of the sample bone, measures the quantity of light transmitted, through the X-ray film, along a plurality of measuring lines perpendicular to the bone axis and spaced at fixed intervals to obtain a light quantity pattern, and smoothes the pattern of the quantity of transmitted light with respect to a direction parallel to the bone axis to compose a pattern to improve repeatability. In the case of the long tubular bone, since the pattern along the bone axis shifts, i.e., the width of the bone changes, only a small amount, the change of the BMD (bone mineral density) attributable to a distortion of the pattern when composing the pattern is small. However, a shift in a pattern of the quantity of transmitted light along the bone axis is large when measuring a cancellate bone such as the end portion of the radius. Therefore, when the pattern covers for a wide area and includes a plurality of measuring lines, the pattern can be distorted greatly and the BMD may not be accurately determined (FIGS. 19A and 10B).
The known bone morphometric methods are applied, in most cases, to the measurement of a typical cortical bone which is isolated from other bones and then the measurement is scarcely affected by the nearby bones and cartilage.
However, when another bone (such as an ulna) exists near the sample bone (such as a radius) and the measurement is liable to be affected by the nearby bone as shown in FIG. 27 and when the sample bone has much cancellate bone as shown in FIG. 28, the transmitted radiation forms a complicated BMD pattern and the measurement is liable to be affected by other bones and soft tissues. Therefore, the known bone morphometric methods are unable to measure the sample bone accurately when the BMD pattern of the transmitted radiation is complicated.